1. Field of the Invention
This invention relates to a method for regenerating and recycling pickle acid used for cleaning zirconium, zirconium metal alloys, hafnium, and hafnium metal alloys. More specifically, this invention relates to a method of removing ZrF.sub.4 from the spent pickle acid, using NaF to precipitate Na.sub.2 ZrF.sub.6. The pickle acid is then bulked by adding HF and HNO.sub.3 to the level required and recycled. The process steps and chemistry are the same for hafnium.
2. Description of the Prior Art
Basically what occurs in the process of pickling of zirconium and zirconium metal alloys is as follows: The pickle acid used for pickling zirconium and its alloys comprises HF and HNO.sub.3. Of the two types of acid present in the pickle acid solution, only hydrofluoric acid reacts with the zirconium metal. Among other functions, the nitric acid primarily reacts with tin which is a common component in the zirconium alloy and also is a source of hydrogen ion which accelerates the HF attack. As the zirconium is eaten away by the HF, the tin is simultaneously removed by the HNO.sub.3. If the HNO.sub.3 was not present, a layer of tin would be left behind on the surface of the zirconium metal and the pickling reaction would be suppressed. Since tin is present in only very small amounts in zirconium alloys, the amount of HNO.sub.3 consumed during the course of pickling is very low compared to the amount of HF consumed.
In terms of a chemical reaction, the following occurs during pickling: 4 HF molecules react with 1 Zr metal atom to form 1 ZrF.sub.4 (zirconium fluoride) molecule and 2 hydrogen gas molecules, i.e.,
______________________________________ 4HF + 1 Zr forms 1 ZrF.sub.4 + 2 H.sub.2 (gas) dissolved in Zirconium this dissolves this bubbles out pickle acid metal into the pickle of the pickling acid and will be acid during removed by the pickling recovery process ______________________________________
At the time that a tank of pickle acid is spent and ready to be dumped, the following substances are present in the acid:
(1) Residual HF which has not been consumed by pickling. PA1 (2) Nitric acid, a small fraction of which has been consumed. PA1 (3) Dissolved ZrF.sub.4 (zirconium fluoride). PA1 (4) Other impurities such as dissolved tin which reacted with the nitric acid.
This spent pickle acid must be disposed of in some way since as it is it is unusable. This presents a pollution problem since it cannot be just dumped. Presently, the spent pickle acid is neutralized with lime. This requires a large expenditure for lime along with there being no saving of any acid.
It has been known to reclaim HF-HNO.sub.3 pickle acid liquors by distillation or by precipitation with barium compounds. The first method disclosed in an article entitled "Mixed Acid Pickle Waste," Metal Finishing, May 1963, discloses a method of distilling HF-HNO.sub.3 from a liquor which has heavy sulphuric acid additions. These pickling acid solutions resulted from the treatment of pickling steel. The distilling method presents problems in that pickle acid at high temperature is extremely corrosive and it is necessary to use protected equipment which is extremely expensive. This, along with the expense of the large amount of energy required for this process, prohibits its use in practice.
The barium salt method, which is disclosed in USAEC Document No. IDO-14511, is also an expensive method in that these barium salts are expensive. This method, however, produces a product which is not easily recyclable in that it has not as yet been determined if barium qualifies as an additive which may be present in the pickle acid during pickling.
It is apparent that there is a need in the zirconium and hafnium industry for an inexpensive, workable process for regeneration and recycling HF-HNO.sub.3 pickle acid solutions.